The present invention relates to a laminated type piezoelectric actuator having a structure of an integral laminated body obtained by alternately laminating a thin platelike piezoelectric ceramic piece and a thin filmlike internal electrode, and more particularly, to a laminated type piezoelectric actuator having a structure in which an internal electrode thin film is alternately laminated over substantially the entire surface of a piezoelectric ceramic piece.
The laminated type piezoelectric actuator of this type makes it possible to enhance the strength of the electric field that is applied to each layer of the piezoelectric ceramic pieces, and moreover, the displacement due to elongation or concentration of the ceramic accompanying the piezoelectric effect for each layer is accumulated, so that a large displacement can be generated for a low voltage, and the actuator is therefore well suited for miniaturization. In particular, the laminated type piezoelectric actuators, which are disclosed in the U.S. Pat. Nos. 4,523,121 and 4,681,667 granted to the same assignee as in the present invention, have approximately the same shape for the piezoelectric ceramic piece and the internal electrode film that are laminated therebetween, and have a structure in which a uniform electric field is applied to the entirety of the piezoelectric ceramic piece. Accordingly, each of the piezoelectric pieces in an actuator with the above-mentioned structure expands or contracts throughout the actuator without generating inactivated parts, so that there are no problems with respect to an uneven expansion or contraction of the piezoelectric ceramic piece due to the inactivated part. Further, the piezoelectric ceramic piece is not damaged by the generation of a stress at the boundary of the activated part and the inactivated part. Therefore, the laminated type piezoelectric actuators disclosed in the aforementioned two U.S. patents are used for a printing head for an impact printer, a small-sized relay, or the like for which small-size and high-speed repeated driving are required.
The reliability of this type of actuator for the above-mentioned usage is high, and the usage tends therefore to be further expanded. It has been observed, however, that the deterioration of the device will appear sooner depending upon the conditions of use. For example, when the driving voltage is exactly or approximately the voltage of a direct current such as in an actuator for closing or opening a mass flow controller valve used for the manufacture of semiconductor integrated circuits, the service life sometimes remains only at about 1000 hours. When the installation place of the actuator is in an environment of high humidity, the life will further be shortened. For example, the result of a humidity resistance on-load life test, in which a maximum rated dc voltage of 150 V (an electric field strength in the piezoelectric ceramic piece being about 15,000 V/cm) is applied continuously, in an atmosphere having a temperature of 40.degree. C. and a relative humidity of 90 to 95%, to a conventional laminated type piezoelectric actuator with a length of 20 mm and a cross sectional area perpendicular to the direction of lamination of 5 mm.times.5 mm, formed into a laminated body by alternately laminating a piezoelectric ceramic piece consisting of lead titanate-zirconate and an internal electrode consisting of a silver-palladium alloy, the mean time to failure (MTTF) was about 500 hours. This value barely satisfies the need of the user who drives the laminated type piezoelectric actuator, but it is not a value with sufficient margin. Ordinarily, a life which is more than ten times the catalog value is desirable.
As a result of observation and analysis of samples that developed failures in the above-mentioned humidity resistance on-load life test, the present applicant found that these failures are caused by the short-circuiting generated between the internal electrodes via microcracks in the outer periphery of the ceramic pieces that are exposed to the atmosphere on the side faces of the laminate. Further, it was observed that the short-circuiting between the internal electrodes was especially concentrated in the vicinity of the four corners or edges of the laminate that run in parallel with the lamination direction of the laminate.